Lunar Lava Tube Dimensions

The following figures are suggested when producing models for lava tube prospecting and use. By standardizing the figures used in models, this will ease the comparison of different models. This document is also a useful repository where people working in this field can find the required information quickly.

Some of the figures are estimates, and are marked as such. They should be treated with caution, and replaced with better figures as they become available.

Unless otherwise stated, all figures come from the Lunar Sourcebook and NASA's Handbook of Lunar Materials.

Rille & Tube Geomorphology

Individual lava flows are thin. They average of 60 feet (20m), some are less than 30 feet (10m) thick. Therefore, they cool quickly, but the viscosity of the melt is very low, so long flows can be found.

Sinuous rilles begin at craters (or crater-like landforms) and fade downslope in smooth mare surface. They occur mainly near basin margins.

Sizes: (from Lunar Orbiter photographs)

Width: tens of feet to 2 miles (tens of meters to 3 km)

Length: a few miles to 200 miles (few km to 300 km)

Depth: mean of about 300 feet (100 m)

Terrestrial tubes form by the build-up of levees on either side of a lava channel. Levees eventually build up to form roofs.

On Earth, subsequent widening and deepening of the tubes is sometimes observed (due to the melting of the roofs and floor). On the moon, widths bridging more than 300 feet (100 meters) have been observed on Lunar Orbiter photographs.

Physical Data for Modeling

Regolith

Density: 1.50 +/- 0.05 (for the top 10 cm) to 1.77 +/- 0.07 g/cm³ (Source: returned samples)

Thickness: 3-20 m at sampled locations. Sampled locations: Apollo (Seismic Refraction) and selected photograph measurements (Orbiter).

P-Wave Velocity: Vp = 92-114 m/s (mean = 105 m/s) S-Wave Velocity: Vs ~ 30 m/s (estimate based on loose sand)

Seismic Q: Approx. 200 @ Vp=105 m/s. For comparison, terrestrial volcanic dust, Q < 5

Vp, Q, and regolith thicknesses come from the Active Seismic Experiments taken on the later Apollo Landers.

Mare Basalt

Density: 2.8 g/cm³ (estimate based on terrestrial basalt)

P-Wave Velocity: Vp ~ 5 km/s (ultrasound, < 1000^oC on cooled re-melt of returned samples)

S-Wave Velocity: Vs ~ 6.4 km/s (estimate based on German basalt)

Thermal conductivity: K=6x10^-3 (at 0^oC, no units given; returned samples)

Electrical data

DC Electrical Conductivity: 10x10^-14 mho/m (regolith) to 10x10^-9 mho/m (basalt) at 300 K in the dark.

In sunlight, conductivity experiences approximately 10x10⁶ times increase.

Relative dielectric permittivity K' = 1.9j^p where p = density in g/cm3

The above data comes from returned samples.

Estimates are based on typical data given in Dobrin M, Savit C, Introduction to Geophysical Prospecting (4th ed).

Comments

On Earth, conductivity surveys use AC to avoid electrolytic effects. As there's no water on the moon, these shouldn't exist, so DC and AC conductivities should be the same.

Lava Tube Modeling

As no clear lava tubes have been found and measured, I propose the following dimensions for use as a standard model for the Artemis Project studies. These are based in part on observed dimensions of sinuous rilles.

                Small Lava Tubes        Large Lava Tubes
   Length       undefined; very long    undefined; very long             
   Width        60 ft (20 m)            1000 ft (300 m)
   Height       45 ft (15 m)             300 ft (100 m)
   Depth        60 ft (20 m)            1500 ft (500 m)

Length could be several miles. The inifinite length assumption is of more use when creating synthetic responses to various geophysical techniques.

Depth is the distance from the surface of the moon to the ceiling of the lava tube. Of these, the depths are particularly rough and ready.

Additional References to Lava Tubes in the Artemis Data Book

How Lava Tubes Form
Section 8.4. Oregon Moonbase.
Uses for Lunar Lava Tubes